1. Field of the Invention
The present invention relates generally to a radio wave prediction technique using an urban canyon model. The invention is particularly useful in wireless communication system design. 2. Description of the Related Art
In a digital microcellular communication system, repeater antennas are distributed throughout a geographical coverage area, particularly an urban area, to communicate directly with wireless communication devices. The repeater antennas are typically hard wired to a main base station (BTS) serving a cell through copper cables, optic cables, or optic waveguides. An important consideration in the design of a microcellular system is where to place these antennas to prevent the occurrence of dead zones where insufficient signal strength is present. A dead zone may be caused by multiple reflections off buildings, etc., that converge at a specific location to cause the signal the fade in and out.
Empirical approaches may be used to optimize the placement of the repeater antennas to minimize performance degradation caused by multiple reflections. However, these approaches are both costly and time-consuming. As such, it is desirable to employ a method for modeling and predicting radio frequency (RF) propagation in the urban environment to arrive at suitable antenna locations. One such model is referred to as an urban canyon model, which defines a canyon formed in the space between a pair of buildings and the ground. The buildings and the ground are all assumed to be lossy dielectrics. A transmitting antenna and receiving antenna are assumed to be standing perpendicular to the ground surface. The RF energy transmitted produces a multiplicity of reflection waves off the buildings and ground. If the propagation pathways of the radio wave from the transmitting antenna to the receiving antenna are known, the reflection coefficients at the respective reflection points may be obtained. A number representing how many times the reflections have occurred in the propagation pathways of the respective reflection waves can also be found. For this purpose, an image technique is employed.
FIG. 1 illustrates the environment of the prior art urban canyon model. As illustrated, a straight road including a ground 3, a building #11, and a building #22 are modeled as forming a dielectric canyon 10. Permittivities (xcex51, xcex52, xcex5g) and permeabilities (xcexc1, xcexc2, xcexcg) are assigned for the respective media of building #1, building #2, and the ground as indicated in FIG. 1. Within the canyon is a transmitting antenna 4 with three dimensional coordinates (xt, yt, zt) and a receiving antenna 5 with coordinates (xr, yr, zr). The radio waves (i.e., rays) emanating from transmitting antenna 4 are assumed to be radiated in all directions. One of the radio waves is a direct wave reaching the receiving antenna directly without any reflection. Other radio waves are multiple reflection waves reaching the receiving antenna by reflecting off one or more wall surfaces of the two buildings 1,2 and the ground surface 3. The image technique is adopted to find the exact points on the wall surfaces and/or the ground at which the multiple reflection waves are reflected.
It is assumed in FIG. 1 that the surfaces of the two buildings are infinite in the y and z directions, and the ground is infinite in they direction. This assumption is allowable because the sizes of the respective reflection surfaces are much larger than the wavelengths of the transmitted radio waves. On account of this, image antennas are assumed to be infinitely generated upon the two surfaces of the buildings 1, 2. Other image antennas are generated beneath the ground. Each image antenna, whether above or below the ground, is intended to correspond to a reflection off one of the buildings or off the ground surface; the location of each image antenna depends on the location and direction of its corresponding reflection ray. Once all image antennas are defined, the received power at the receiving antenna 5 can be computed using a free space model that sums the RF energy contributions from the various image antennas. An equation defining the received power caused by the direct waves received at the receiving antenna 5 and the multiple reflection waves, is:                               P          r                =                                                            P                t                            ⁡                              (                                  λ                                      4                    ⁢                    π                                                  )                                      2                    ⁢                                    "LeftBracketingBar"                                                ∑                                      n                    =                    0                                    ∞                                ⁢                                  xe2x80x83                                ⁢                                                      G                    n                                    ⁢                                      R                    n                                    ⁢                                                            ⅇ                                              j                        ⁢                                                  xe2x80x83                                                ⁢                                                  kr                          n                                                                                                            r                      n                                                                                  "RightBracketingBar"                        2                                              EQ        .                  xe2x80x83                ⁢        1            
where, Pt is the transmitting power, xcex is the wavelength of the radio wave, k is the wave number (2 xcfx80/xcex), n is the number of propagation pathways, Gn is the square root of the gain product of the transmitting and receiving antennas in the nth propagation pathway, Rn is a pathway reflection coefficient, and rn is the distance of the propagation pathway between the transmitting antenna 4 and the nth receiving image antenna. If n=0, then this indicates the direct wave; all other values of n indicate reflection waves. Considering the directivities and beamwidths of the transmitting and receiving antennas, the value of Gn may be varied depending on the relative locations of the transmitting and receiving antennas. The parameter Rn represents the product of the reflection coefficients of the waves reflected on the surfaces of buildings 1,2 and/or ground 3, multiplied by the reflection counts. EQ. 1 assumes that the radio waves are all vertically polarized (xcex8-direction). Only the radiation field strength, and not the polarization effect, is taken into account.
FIGS. 2A and 2B illustrate a prior art procedure of generating and numbering the image antennas. FIG. 2A illustrates the generation of the image antennas and x-coordinates, and FIG. 2B illustrates the numbering of the image antennas.
The following is an explanation of a prior art algorithm which finds the propagation pathways of the direct waves and the multiple reflection waves existing in the canyon model.
Referring still to FIG. 1, image antennas corresponding to the reflection waves off the wall surfaces are generated because of the two dielectric surfaces, that is, the wall surfaces of the buildings. Image antennas beneath the ground surface correspond to reflection waves that include a reflection off the ground surface. Rnv indicates the image receiving antennas generated due to reflection off the surfaces of the two buildings 1, 2 and the ground surface 3, where n is the number of a particular image antenna, and v is a number representing whether that image antenna is above or below the ground surface. For an image antenna above the ground surface, the number v is assigned xe2x80x9c0xe2x80x9d, and for an image antenna beneath the ground surface, v is assigned xe2x80x9c1xe2x80x9d. Therefore, the nth image receiving antenna above the ground surface is designated as Rno and the nth image receiving antenna beneath the ground surface is designated as Rn1.
The indefinite image antennas generated by the surfaces of the two buildings are numbered as follows:
Actual receiving antenna 5 is assigned n=0, thereby being denoted by R00. Image antennas generated by the reflections from the walls of both buildings are numbered as follows: those residing in the x less than 0 area are assigned odd numbers, and those residing in the x greater than 0 area are assigned even numbers, in sequence. FIG. 2A shows the numbered antennas, and the numbering rule is illustrated by the square wave of FIG. 2B. For each propagation pathway to be considered, the transmitting antenna is assumed to generate two image antennas R10 and R20. Images R10 and R20 correspond to reflections off the left and right building surfaces, respectively. Image antennas generated from R10 are denoted with the numbers in the lower part of the square wave, and image antennas generated from R20 are denoted with the numbers in the upper part of the square wave. Thus, image R10 (i.e., R1 in FIG. 2A or xe2x80x9c1xe2x80x9d in FIG. 2B) produces a ray that reflects off the right building to generate image R40 (i.e., R4 in FIG. 2A or xe2x80x9c4xe2x80x9d in FIG. 2B). Image R40 then produces image R50, and so forth, until the RF energy reaches the receiving antenna through continued multiple reflections. Likewise, image R20 produces images R30, R60, etc. It is noted here that the term xe2x80x9cpropagation pathwayxe2x80x9d, as used herein, means any path in which RF energy from the transmitting antenna can reach the receiving antenna, whether or not reflections occur. Thus, for instance, the direct path from the transmitting antenna to the receiving antenna defines one propagation pathway; another propagation pathway is the path in which RF energy from transmitting antenna 5 reflects off a single surface and then reaches the receiving antenna 4; yet another pathway includes a reflection off only two surfaces to reach receiving antenna 4; and so forth.
When computing the receiving power using EQ. 1, RF energy is assumed to emanate from the respective image antennas and arrive at the receiving antenna 5. The total counts of the reflections generated because of buildings 1 and 2 must be known. The odd image antennas R10, R30, R50, R70, etc. represent rays that start from the transmitting antenna 4, and are first reflected off building #1, pass respective remaining pathways, and then arrive at the receiving antenna 5.
On the contrary, for even n, the image antennas R20, R40, R60, R80, etc. represent rays that start from the transmitting antenna 4, and are first reflected off building #2, pass respective remaining pathways, and then arrive at the receiving antenna 5. In the square wave diagram of FIG. 2B, the vertically aligned antenna numbers, that is, {0}, {1,2}, {3,4}, {5,6}, etc. have common reflection counts mn=0, 1, 2, 3, etc., in sequence. For example, image antennas 1 and 2 each have a reflection count of 1; image antennas 3 and 4 each have a reflection count of 2; and so forth. The general equation for the reflection count of the nth image antenna is:                               m          n                =                                            (                                                2                  ⁢                  n                                +                1                            )                        +                                          (                                  -                  1                                )                                            n                +                1                                              4                                    EQ        .                  xe2x80x83                ⁢        2            
where, n=0, 1, 2, 3, etc.
The reflection process of the image antennas beneath the ground surface is identical to the case of the image antennas on the ground surface, and includes one more ground surface reflections.
From EQ. 1 and EQ. 2, the coordinates (xn, yn, zn) of the (n, v)th image receiving antenna Rnv are:                               x          n                =                                                            (                                  -                  1                                )                                            m                n                                      ⁢                          x              r                                +                                    {                                                                                          (                                              -                        1                                            )                                        n                                    ⁢                                      m                    n                                                  +                                                      1                    +                                                                  (                                                  -                          1                                                )                                                                                              m                          n                                                +                        1                                                                              2                                            }                        ⁢            w                                              EQ        .                  xe2x80x83                ⁢        3                                          y          n                =                  y          r                                    xe2x80x83                                          z          v                =                                            (                              -                1                            )                        v                    ⁢                      z            r                                              xe2x80x83            
where, mn is defined by EQ. 2, yr and Zr are the respective y and z coordinates of receiving antenna 5 and W is the width of the road between the two buildings. Also, to determine how many image antennas are generated for each propagation pathway, it is necessary to determine the number of reflections that occur between the two buildings. By generating image antennas corresponding to the respective multiple reflection waves, the entire space is replaced with free space without obstacles. As such, the receiving power equation used in free space may be employed.
This numbering method is meritorious in that it facilitates finding the indefinite multiple propagation pathways. Because of the long distance between the transmitting and receiving antennas, it is assumed that only the vertical component of the electric field exists. Therefore, when reflection occurs in the canyon model, the waves reflected off the building are assumed to be vertically polarized whereas the waves reflecting off the ground are assumed to be horizontally polarized, and the gain of the transmitting antenna and the receiving antenna is, assuming a dipole antenna, fixed as 1.64 dBi.
In the actual urban environment, however, although the transmitting antenna is fixed vertical to the ground surface, the polarization direction of the receiving antenna may be freely varied by the user. That is, in the canyon model, not only the vertical component to the ground surface, but also the horizontal component, though it is weak, exist, and this horizontal component affects the receiving power. But, in the prior art, because the electric field is regarded as a scalar component, the receiving power affected by the variation of the polarization direction of the antenna cannot be found.
Another prior art method for predicting RF propagation, which finds multiple propagation pathways by considering reflections that are generated within the propagation pathways in a building, is disclosed in U.S. Pat. No. 5,450,615 entitled xe2x80x9cPrediction of Indoor Electromagnetic Wave Propagation for Wireless Indoor Systemsxe2x80x9d. In this patent, an imaging method is used to predict RF propagation within a structure. Each reflective surface is associated with a reflection and transmission coefficient. Reference transmitter and receiver locations are assumed, with a reflection path traced backwards from each receiver location to the reflective surfaces to generate images. While this technique may be useful for predicting indoor propagation, its utility for predicting urban environment outdoor propagation in a computationally efficient manner is questionable.
It is an object of the present invention to provide a method for predicting the characteristics of wave propagation by considering the electric field as not a scalar component but a vector component so as to be applied to actual urban conditions and finding the unit vector of the direction to which the radio waves start from the transmitting antenna.
It is another object of the present invention to provide a method for computing the receiving power considering the directivity and the polarization of the transmitting antenna and the receiving antenna within an urban model.
It is another object of the present invention to provide a method for finding the coordinates of the first reflection point in order to find the propagation pathways corresponding to the respective image antennas.
It is another object of the present invention to provide a method for considering the polarization of the receiving antenna by obtaining the product between the polarization vector of the receiving antennas and the radio wave reaching the receiving antenna.
In an illustrative embodiment of the invention, a method for predicting the characteristics of wave propagation in an urban canyon model considering the effect of polarization includes the steps of: numbering a plurality of image antennas corresponding to reflections between transmitting and receiving antennas in an urban canyon; determining the propagation paths corresponding to the respective image antennas; computing the respective first reflection points of the propagation paths corresponding to the image antennas; computing the respective reflection electric field vectors of the propagation paths corresponding to the image antennas; and finding the total received power of the receiving antennas through the reflection electric field vectors of the propagation paths and the unit polarization vector of the receiving antenna. Advantageously, accurate fading effects can be obtained by assuming a sufficiently large number of propagation pathways. In addition, the distribution of the receiving power can be obtained for a varying polarization of the receiving antenna.